1. Field of the Invention
The invention relates to a failure diagnostic apparatus and a failure diagnostic method for a gas concentration detecting apparatus. More particularly, the invention relates to a failure diagnostic apparatus and a failure diagnostic method which are suitable for detecting disconnection in a gas concentration detecting apparatus which detects the concentration of NOx contained in exhaust gas from an internal combustion engine.
2. Description of the Related Art
As a gas concentration detecting apparatus which detects the concentration of NOx contained in exhaust gas from an internal combustion engine, for example, an apparatus according to related art disclosed in Japanese Patent Laid-Open Publication No. 2002-202285 is known. The apparatus according to related art includes a pump cell which discharges oxygen from exhaust gas flowing into a gas detection chamber, and a monitor cell and a sensor cell which are disposed downstream of the pump cell.
The monitor cell discharges oxygen which remains in the gas detection chamber after the pump cell discharges oxygen, thereby generating an electric current corresponding to the concentration of oxygen. Meanwhile, the censor cell decomposes NOx contained in the exhaust gas into nitrogen and oxygen after the pump cell discharges oxygen, and discharges the resultant oxygen and the oxygen that remains in the exhaust gas, thereby generating an electric current.
According to the aforementioned configuration, the value of the electric current flowing in the monitor cell corresponds to the concentration of the oxygen which cannot be discharged by the pump cell. Also, the value of the electric current flowing in the sensor cell corresponds to the sum of the concentration of the oxygen which cannot be discharged by the pump cell and the concentration of the oxygen generated by decomposition of NOx. Therefore, by subtracting the value of the electric current flowing in the monitor cell from the value of the electric current flowing in the sensor cell, it is possible to detect the concentration of oxygen generated as a result of decomposition of NOx. The concentration of the oxygen thus detected corresponds to the concentration of NOx that exists in the exhaust gas. Accordingly, the aforementioned gas concentration detecting apparatus according to related art can accurately detect the concentration of NOx in the exhaust gas.
The aforementioned gas concentration detecting apparatus according to related art is used for emission control of an internal combustion engine, or the like. When output of the gas concentration detecting apparatus is used for emission control or the like, it is desirable to early detect abnormality of the gas concentration detecting apparatus in order to maintain a desired emission characteristics. However, a method for early detecting disconnection in the gas concentration detecting apparatus of this type has not been established.